A Search for Water Vaporization on Ceres

There are hints that the dwarf planet (1) Ceres may contain a large amount of water ice. Some models and previous observations suggest that ice could be close enough to the surface to create a flux of water outward through the regolith. This work aims to confirm a previous detection of OH emission off the northern limb of Ceres with the International Ultraviolet Explorer (IUE). Such emission would be evidence of water molecules escaping from the dwarf planet. We used the Ultraviolet and Visual Echelle Spectrograph of the Very Large Telescope to obtain spectra off the northern and southern limbs of Ceres at several epochs. These spectra cover the 307-312 nm wavelength range corresponding to the OH (0,0) emission band, which is the brightest band of this radical, well known in the cometary spectra. These new observations, five times more sensitive than those from IUE, did not permit detection of OH around Ceres. We derive an upper limit for the water production of about {approx}7 x 10{sup 25} molecules s{sup -1} and estimate the minimum thickness of the dust surface layer above the water ice layer (if present) to be about 20 m.